1. Field of the Invention
The present invention relates to electronics and electrical systems. More specifically, the present invention relates to GPS (global positioning system) receivers.
2. Description of the Related Art
The primary function of a GPS receiver is to determine the position and velocity of its antenna and GPS system time from signals transmitted by a constellation of satellites. To achieve this end, a GPS receiver must first acquire a coarse/acquisition (C/A) signal when the receiver's initial position, velocity, and estimate of GPS time are not known sufficiently well. After the C/A code signal is acquired, the receiver must perform a number of steps to correctly estimate the transmit time of the signal, which is necessary to compute a navigation fix. These steps require a certain amount of time that add to the receiver's time-to-first-fix (TTFF). A fast TTFF is crucial to many military as well as commercial applications.
The most time consuming steps that contribute to a receiver's TTFF are bit synchronization, frame synchronization, and demodulating the hand-over word (HOW) from the signals transmitted by the satellites. These steps are typically performed independently and in parallel for each satellite being tracked, using one demodulator channel per satellite. Thus for a particular satellite, bit synchronization must be achieved by successively trying different channel timing setups until the correct one is found. Prior art methods ordinarily used to determine the correct timing tend to be sensitive to the frequency of bit transitions during the data collection process and are also susceptible to the effects of interference or low signal strength. Consequently, if there are too few bit transitions while a given timing setup is being tested, or the signal strength fluctuates, the wrong timing hypothesis may be accepted. When this happens, the TTFF may be extended by a considerable amount of time.
Following bit synchronization; the receiver must determine where individual words and frames start within the navigation message. The data bits transmitted by each satellite are grouped into 30-bit words, which in turn are organized into a ten-word subframe. The first word of every subframe contains a pattern that allows the receiver to verify that it has correctly determined the beginning of words and subframes. The process of locating this pattern is generally referred to as frame synchronization, and since the data bits are transmitted at a 50 Hz rate, this operation can take up to six seconds to complete.
Demodulating the HOW is usually accomplished after the frame synchronization process completes. The HOW contains a 17-bit quantity called the time-of-week (TOW) count, which corresponds to the time of the week at the start of the next subframe. Once the receiver obtains the TOW count, it is able to determine the time of the week associated with every C/A chip. The time associated with any given chip of the C/A code sequence is referred to as transmit time and is heeded to compute a navigation fix.
Bit synchronization, frame synchronization, and obtaining the time-of-week count from the HOW are necessary because the C/A sequence repeats once per millisecond. When tracking the C/A code phase, the receiver can very accurately estimate the transmit time within a particular millisecond, however, it cannot determine which millisecond relative to a certain epoch. By performing the steps described previously, this ambiguity can be resolved by the receiver. As stated earlier, in the prior art, these steps (among others) are performed in parallel for each satellite being tracked and therefore require one demodulator channel per satellite. While straightforward in concept, this approach does not make the most efficient use of available channel resources to minimize TTFF.
Hence, a need exists in the art for an improved GPS receiver that offers a shorter TTFF than conventional GPS receivers.